# The Science Behind the Collapse of WTC7

On September 11^{th} 2001, three buildings descended to the ground, each in a very short space of time. For each of these buildings, it has been demonstrated by careful measurements conducted on the copious video footage that, for a large part of the descent, accelerations indistinguishable from that of free-fall were achieved.

The fact that this is impossible for a structure undergoing a gravity collapse has been obvious to most scientists for years. Even Shayam Sunder, NIST spokesperson, has admitted as much, as you can see in this clip (ffwd to 0m20s).

Up until now, however, the proof of this has been inaccessible to the layperson, especially those who are easily intimidated by physics. In this article I will explain why such a collapse is impossible in simple terms and illustrate the point with a couple of everyday examples. In this way I hope that the proof will become obvious to all.

I’m going to focus on WTC7 because it is easier to see the mechanics of what’s going on from the video clip. Also, as it wasn’t hit by a plane, this removes one unnecessary distraction from the discussion.

## Newton

In order to understand the collapse, we just need to be familiar with one simple rule of physics: Newton’s Third Law. The best way to understand this is with a simple example, but, just to be thorough, let’s start with the scientific definition. Newton’s Third Law says that, when any body (let’s call it “A”) exerts a force on another body (“B”), then “B” exerts an equal and opposite force on “A”.

So let’s look at a simple example.

Imagine that you have a large receptacle full of trash and that you wish to compact the contents so you can get some more trash in. You put your foot on top of the trash and press down. Initially this is easy, but, bit by bit, it gets more difficult. Eventually you find you have to use your whole weight to compress it further and, eventually, even this is not enough and things come to a stop. You can’t compress it any more unless you grab on to something to get some leverage or get a friend to help.

## A Trashy Analysis

Why is this? Well, here is where Newton’s Third Law comes in. Initially the trash, being loosely compressed, does not require much force to crush it. The force needed to compress it is exerted on it by your foot. At the same time, due to Newton’s Third Law, an equal and opposite force (the “Normal Contact Force”) acts upwards on your foot. This is because the top of the trash pile and the bottom of your foot are in contact and your foot (body “A”) is exerting a force on the trash (body “B”). Your muscles work to balance this upward force and it is by this exertion that you estimate how “hard” it is to compress the trash.

Notice that before your foot first touches the surface of the trash, pushing your foot down is very easy because you are just pushing air. When it hits the trash there is a little jolt because now there is the Normal Contact Force (NCF) acting up against your foot.

After a while, the rubbish becomes more compressed. This means that it now has more structural integrity, and it is this that makes it harder to compress it further. More force needs to be exerted and, by Newton’s Third Law, the NCF on your foot increases with it. Your muscle needs to exert more force to balance the NCF and so it seems “harder” to compress.

Now let’s try and use what we have learned and apply it to WTC7. In the course of this, I am going to make some assumptions just to make the thought experiment flow in as simple a way as possible. Some of the assumptions may seem unusual, so I am going to annotate them as we go and I will show at the end why they do not matter.

## The Real Thing

Let’s start by looking at the clip of the building. Look at the clip and consider the first point of the main collapse (ffwd to 0m45s). We can see that the visible mass of the building moves down in one lump and we cannot see any destruction, at least not for a while. This means that the “zone of destruction” (ZOD) is somewhere near the bottom and does not move around much. Let’s assume (assumption1) that it is on the first floor and is fixed (assumption2).

Now we have to imagine the descent starting somehow, so let’s imagine that the first floor is removed (assumption3). I know this seems a little bit of an odd thing to do, so don’t forget to look at the notes at the end to see why we don’t need to worry about this.

The upper part of the building (let’s call it the “head” which, at this stage, consists of floors 2 through 47) will now start falling downwards under the force of gravity towards the stationary lower section (the “stump” which just consists of the ground floor). We can see this happening in the clip. It falls with free-fall acceleration because gravity is the only force acting. See fig1.

**Fig1**

After a while the lower part of the second floor will be about to hit the upper part of the ground floor. At this point, let’s imagine two different scenarios.

## Removal

In the first, let’s imagine that the second floor is removed from the picture (somehow). In this case the head will continue to fall downwards under gravity. Since gravity is still the only force acting, it will continue to accelerate at free-fall. In order for the ZOD to stay still, the second floor would have to be removed at the exact point in time that it reaches the position where the first floor used to be a few moments ago. So timing of the removal would be crucial. After this, the head would accelerate further and soon the third floor would be the next candidate for removal. Iterate this through all 47 floors and we will see a free-fall descent of the whole building.

Before we move on to the second scenario, let’s consider what would have happened if the timing of the removal of the second floor was mismanaged. If it were removed too early, then the ZOD would move upwards because the removal would be happening more quickly that the building was falling. If this were to continue, we would eventually see to ZOD in the clip – which we don’t. If, on the other hand, the second floor were to be removed too late, then the head would crash into the stump. We will cover what would happen in this case in the second scenario.

## Crumpling

So let’s move to the second scenario then. In this case, we imagine what would happen if this “removal” (whatever that, in fact, means) does not take place. In this case, the bottom of the second floor (that is to say the bottom of the head) will strike the top of the ground floor (the top of the stump) just like your foot struck the top of the trash. See fig2.

**Fig2**

Just as with your foot on the trash, the force will be exerted downwards on the stump (the trash) and the NCF (equal and opposite) will act upwards on the head (your foot). In the diagram, the force is in yellow and the NCF is in red. This NCF (on the head) is called the “retardation” because it acts to slow down the descent of the head. The NCF is in **opposition** to the force of gravity so we would observe a jolt as the forces on the head suddenly changed. In order to calculate the new value for the acceleration of the head, we need to *resolve* (i.e. subtract) these forces. Don’t worry if this seems complicated. The important thing is that now the head will be accelerating at **less than free-fall** thanks to the retardation.

I have a feeling that this retardation is the thing that most people fail to take into account when they look at this issue. But the retardation is a necessary consequence of Newton’s Third Law, and the application of Newton’s Third Law is a necessary consequence of the two bodies’ coming into contact. Most people think of the Law as applying only in static situations. However, there is no such *caveat* and to see it happening here in a dynamic situation is unremarkable.

Going back to the thought experiment, we can see that, now the two bodies are in contact, crumpling will occur just like it did with your foot on the trash. And this crumpling will continue. Just like with your foot, the force necessary to compact the remnants of the lower floors will increase. Eventually, it will increase so much that it will become equal to the weight of the head and we will no longer see acceleration of any kind but, instead, we will observe constant downward velocity of the head. The crumpling force (and thus the retardation) however, will increase still more and, eventually, we will see deceleration and an eventual halt.

This, I think you will agree, is very different from a free-fall descent.

## Collision

Let’s consider another perhaps easier example. Imagine two identical automobiles: a stationary one (A) and another (B) moving at a constant velocity of 10m/s. See fig3.

**Fig3**

Now imagine that B hits A. See fig4.

**Fig4**

After the collision, the crumpled unit that consists of both cars will be moving at 5m/s. In physics terms, this is called “Conservation of Momentum”, but like most practical physics, we can also just view it as common sense. I hope that most people will be able to see this intuitively without a lengthy discussion of momentum.

Let’s imagine that the collision process takes 1s to complete (from the first instant of the impact up to the point at which both vehicles are moving along as one unit). This means that A will have accelerated at 5m/s/s and that B will have **decelerated** by 5m/s/s. During the 1s, the mechanics of the crumpling will have absorbed the change in momentum required to make the new composite object travelling at 5m/s. The important thing is the deceleration of B.

Now this example isn’t very similar to WTC7, so let’s see what can change to make it a better fit. Well, the head was very big as compared to the stump, so let’s make B a big truck. Also, the stump is not on wheels, so let’s remove the wheels from A and embed it in concrete to anchor it down. The stump cannot go anywhere because there is solid ground underneath it, so let’s build a big concrete wall behind A and assume that it is infinitely strong. Lastly, the head was accelerating (before impact) under gravity, so let’s imagine a robot driver with its foot on the gas. See fig5.

**Fig5**

Now we can see that B would be accelerating until the moment of impact but, after impact, we would see a sharp reduction in acceleration. As crumpling proceeded, the acceleration would decrease more until it became deceleration and eventually the whole thing would come to rest. At the end A and B would still be visible, and would be roughly in the position that A once occupied.

This is not what we observed in the clip of WTC7 at all.

Now imagine what we would see if layers of B were to be removed from the front (of B) before the impact could happen. In this case, the acceleration would continue (because the robot foot is on the gas and there is nothing to slow the truck down) until there was nothing left of B.

This is exactly what we observe in WTC7.

So what can we conclude? Well, Newton’s Third Law has taught us what has to have happened in order to produce the observations we can see in the clip. Each floor of the building would have had to have been “removed” in a very precisely timed and orderly manner so that no real contact ever took place between the head and the stump. If, at any point, there **was** contact, then a NCF would have been produced and that would have resulted in retardation.

No such retardation was observed.

Now watch the physics expert, Dave Chandler from @AE911Truth, explaining the same thing (ffwd to 1m20s).

Now we have worked out what must have happened, we need to propose a mechanism (or model) that could produce this. Specifically, we need to suggest how this “progressive removal” could have happened.

My proposed model is that of controlled demolition, and it is my claim that this model would produce both the “removal” and the “timing” that are required to produce the observations we see. Of course, it may be possible to come up with an alternative model. I am not aware that anyone has done so, but, if anyone has, I would be very interested in analysing it to see if it could more accurately predict the observations we made. This, of course, is the scientific method. All we can say is that controlled demolition is the most likely explanation we have considered so far. We should always keep an open mind in case something new comes along.

All that remains now is to tidy up those loose assumptions:

*Assumption1* (ZOD is at the first floor). It makes no difference if we assume the ZOD starts at any other floor. The stump just starts out being bigger and the head smaller (or vice versa).

*Assumption2* (ZOD is fixed). It is perfectly OK for the ZOD to move around (though not too far, otherwise we would see it in the clip). It just means that the timing for the “removal” would be more difficult to work out, making the chance of a collision more likely. Remember, if there is a collision, then we will not get the results that we have observed.

*Assumption3* (First floor is removed at the start). If this seems weird, imagine just a small section 1m in height being removed. The argument above still stands, the only difference being that the first step happens more quickly. If that’s still weird, imagine this removal being applied to a section that is infinitely small in height. The argument above now happens at the start and there is no jolt. The rest is the same.